1. Field of the Invention
This invention relates to heat vulcanizable polyorganosiloxane elastomer compositions. More particularly, this invention relates to novel polyorganosiloxane compositions that can be cured at elevated temperatures to yield elastomers exhibiting desirable combinations of physical properties such as tensile strength, elongation and tear strength.
2. Description of the Prior Art
The physical properties of polyorganosiloxane elastomers can be improved using a number of methods discussed in the prior art. These methods include addition of inert reinforcing fillers, such as finely divided silica, and/or crosslinking agents that react to form chemical bonds between adjacent polyorganosiloxane molecules.
One of the earliest methods for curing polyorganosiloxanes, for example trimethylsiloxy endblocked polydimethylsiloxanes, employs an organic peroxide such as dibenzoyl peroxide. Dibenzoyl peroxide is an example of a nonvinyl specific peroxide which decomposes relatively rapidly at elevated temperatures forming free radicals that react with the polysiloxane by removal of hydrogen atoms from methyl groups to form free radicals that, in turn, react with one another during the curing reaction to form a covalent bond, thereby joining together two polymer mer units. The sites at which these chemical crosslinks form using nonvinyl specific peroxides are likely 2 combinations of intra and intermolecular crosslinks that exist as clusters of crosslinks. This type of crosslinking is apparently unfavorable for such properties as recovery and/or compression set.
It was subsequently found that certain recovery properties of cured polydiorganosiloxanes can be improved by replacing vinyl-free polydiorganosiloxanes or those containing only terminal vinyl radicals with ones containing nonterminal vinyl radicals at randomly spaced intervals along the polymer molecules. Typically such polymers contain one vinyl radical per 300 to 800 diorganosiloxane units, and are cured using vinyl-specific peroxides, such as 2,5-dimethyl-2,5-di-t-butylperoxyhexane or dicumyl peroxide, that react preferentially with vinyl radicals and introduce one crosslink for each vinyl radical. The cured polydiorganosiloxanes exhibit improved compression set values relative to polymers which do not contain vinyl radicals along the polymer molecule, however the elongation and tear strength of these vinyl-containing polymers following post-curing are not high enough for certain end-use applications, particularly those during which the elastomers are subjected to high stress at elevated temperatures.
U.S. Pat. No. 2,803,619, which issued on Aug. 20, 1957 to Dickmann, teaches improving the compression set of siloxane elastomers by combining a nonalkenyl containing containing polydiorganosiloxane gum with a vinyl-containing polysiloxane containing at least 12 silicon atoms per molecule. The resultant mixture contains one vinyl radical for each 20 to 5,000 silicon atoms and is cured using an organic peroxide. The cured compositions exemplified in this patent exhibit improved compression set values relative to a control prepared without any vinyl-containing polymer, however this improvement is achieved at the expense of maximum elongation, i.e., elongation at break, which decreases from 310 percent for the control to a maximum of 260 percent for a sample containing 10% by weight of vinyl-containing polyorganosiloxane.
In U.S. Pat. No. 2,819,236, which issued on Jan. 7, 1958, Dickmann discloses the preparation of polyorganosiloxane elastomers having durometer values of less than 30 by curing a mixture containing 100 parts by weight of a polydiorganosiloxane gum and from 10 to 35 parts of a fluid containing phenylmethylsiloxane and either alkylvinylsiloxane or phenylvinylsiloxane units. Specified amounts of dimethylsiloxane and dimethylalkyl- or dimethylvinylsiloxane units are also present in the fluid, which exhibits a viscosity of less than 1 m.sup.2 /sec at 25.degree. C. The compositions are cured using an organic peroxide. As in the case of the aforementioned U.S. Pat. No. 2,803,619 the desired property, in this instance a low durometer value, is achieved at the expense of maximum elongation. The one reported maximum elongation value for a post cured composition is 580%. This sample was post cured for 24 hours at a temperature of 250.degree. C.
U.S. Pat. No. 3,652,475, which issued to T. Wada and K. Stoh on Mar. 28, 1972, discloses peroxide-curable polyorganosiloxane compositions containing (a) 100 parts by weight of a polydiorganosiloxane having an average degree of polymerization of at least 3,000 and containing either no vinyl-containing siloxane units or from 0.02 to 0.3 mole percent of vinyl-containing siloxane units, (b) from 1 to 10 parts by weight of a polydiorganosiloxane having an average degree of polymerization of at least 3,000 and containing from 5 to 20 mole percent of vinyl-containing siloxane units and (c) from 0.5 to 5 parts by weight of a polydiorganosiloxane having an average degree of polymerization of from 10 to 1,000 and containing from 5 to 90 mole percent of vinyl-containing siloxane units. The alleged benefits of higher tear strength, resiliency and lower compression set are obtained at the expense of maximum elongation, which is 580% and 530% for the two samples that are post cured for 24 hours at 250.degree. C. Post-curing under these conditions is required for elastomers that are expected to retain their tensile and recovery properties and dimensions during exposure to temperatures above about 150.degree. C.
An objective of this invention is to provide peroxide-curable polyorganosiloxane elastomer compositions that retain useful levels of elongation, tensile strength, tear strength and elastic recovery following exposure to temperatures above 200.degree. C. for extended periods of time. These properties are particularly desirable for elastomer articles that are subjected to stresses such as repeated flexing, tension and friction at temperatures above 100.degree. C. during use. Specific examples of such articles are conveyor belts, drive belts, hoses and tubing connecting moving parts, flexible molds, and seals between a moving part, such as a rotating shaft and a stationary body such as a motor housing.
It has now been discovered that the aforementioned objective can be achieved using the curable elastomer compositions of this invention.